Along with developments of small-sized and high performance portable devices and a broadening of new applications, for example, HEV, there are two limited trends in lithium ion secondary batteries towards high capacity and high output. In any situation, an environment that is significantly increased in the calories generated inside of a battery and deteriorated in heat radiation is created inside of the battery. It is therefore necessary to improve the heat stability of battery materials in order to ensure the safety.
As for the improvement in the thermal stability of battery materials, many studies have been made about active materials involving studies of elemental substitution and a control of powder properties and about electrolytes involving studies of flame retardant solvents including ionic solvents.
However, with regard to a separator, a development of a thinner type is desired, but on the other hand, it is highly desired to improve its heat resistance in the case of attaining the above high capacity. It is difficult to make an improvement of a thinner type while retaining the safety function such as shutdown of the separator. Particularly, there is a problem that shrinkage is increased by raising higher temperatures in the development of a thinner type separator, and it is therefore difficult to make a balance between an improvement in the heat resistance of a separator and a development of a thinner type separator. For example, when a laminate structure is formed by laminating a heat resistant resin such as polypropylene (PP) on polyethylene (PE), thermal stability is improved. However, physical properties such as pore diameter and porosity are greatly changed in relation to the problem concerning each thickness of the both to be stuck and to production method and it is therefore difficult to provide the same performance as conventional separators.
In addition to the above, with regard to a gel polymer battery using a polyethylene (PE) separator, there is a report concerning a method for suppressing shrinkage by retaining the adhesion of a gel electrolyte between the separator and an electrode. However, though the effect for suppressing the thermal shrinkage of a separator by the adhesion is higher than that of an electrolyte battery because a gel polymer is more deteriorated in ion conductivity than the electrolyte battery, the gel polymer battery has a disadvantage in inclusion of a liquid and high-rate discharge and has a difficulty in meeting the performance required in the market.
With regard to the prevention of the thermal shrinkage of a separator by improving the adhesion between the separator and an electrode, it is necessary to use a polymer reduced in solubility in an electrolyte solution to secure adhesion in the electrolyte solution. However, in this case, the polymer layer is reduced in ion conductivity and therefore, a deterioration in battery characteristics is significant. In the case of using a polymer electrolyte which is easily gelled on the other hand, it is easily dissolved in the electrolyte solution and therefore only insufficient adhesion is obtained.
In such a situation, there have been the following patent applications using a polymer having an epoxy group or oxetanyl group: including applications in Patent References 1 and 2 using the polymer as a gel electrolyte, applications in Patent References 3 and 4 using the polymer for preventing short circuits from being developed in a separator and an application in Patent Reference 5 using the polymer to be applied to a separator for adhesion between the separator and an electrode.
An in-battery polymerization system in which all electrolyte solutions contained in a battery cell are gelled is adopted in Patent References 1 and 2 and a method in which an electrolyte solution containing a curing agent is poured on a separator which is coated or impregnated with a polymer having a crosslinking group containing an epoxy group or oxetanyl group in advance to crosslink the polymer to use it as a gel electrolyte is adopted in Patent References 3 and 4, thereby intending to limit the thermal shrinkage of the separator by the reinforcing effect of the crosslinked polymer. However, the former gives rise to the problem concerning a deterioration in ion conductivity, leading to a reduction in charge-discharge characteristics and the latter poses the problem concerning a reduction in adhesion and dissolution of the polymer in the electrolyte solution because the total amount of the polymer is lower for the electrolyte solution, giving rise to the problem that the generation of gas is increased because of storage at high temperatures or the like.
Also, Patent Reference 5 also discloses that a polymer is applied to a separator to bind an electrode with the separator. However, in this method, the polymer is dissolved in the electrolyte solution when the electrolyte solution is poured and therefore sufficient adhesive effect can be scarcely developed.
Some other polymer batteries adopting the in-battery polymerization system are disclosed. However, as mentioned above, the in-battery fully gelled type battery has poor ion conductivity and it is difficult to create sufficient charge-discharge performance in, particularly, a high-capacity battery. Also, as disclosed in Patent References 3, 4 and 5, a method is disclosed in which a crosslinking polymer is applied to a separator and an electrolyte solution containing a crosslinking agent is poured to obtain an adhesive effect. However, because it is necessary to use a polymer having high affinity to the electrolyte solution in a small amount (thin film) to attain the charge-discharge performance, it is dissolved in the electrolyte solution before the electrolyte solution containing a crosslinking agent is poured, posing the problem that only insufficient adhesive effect is obtained and the dissolved polymer adversely affects the charge-discharge performance of the battery.
Patent Reference 1: Publication of JP-A No. 2001-176555
Patent Reference 2: Publication of JP-A No. 2002-110245
Patent Reference 3: Publication of JP-A No. 2003-142158
Patent Reference 4: Publication of JP-A No. 2003-142159
Patent Reference 5: Publication of JP-A No. 2004-185920